The xeroprinting process employs a printing plate, commonly referred to as a "master", made by creating a pattern of insulating material, i.e., an image, on the surface of a grounded conductive substrate. The master is exposed to an electrostatic field, e.g., by a corona discharge, that imposes an electrostatic charge on the surface of the master. That portion of the master bearing the insulating material retains the charge, while the charge on the remainder of the master is discharged through the grounded conductive substrate. A latent image of electrostatic charge is formed on the insulating material. The image is subsequently developed with oppositely charged particles commonly referred to as a "toner". The toner is transferred, e.g., by electrostatic or other means, to another surface, e.g., a paper or polymeric film, where it is fused, i.e., fixed, to reproduce the image of the master. Since the insulating material is permanent, or at least persistent, multiple copies can be made by repeating the charging, toning, and transfer steps.
Riesenfeld et al., U.S. Pat. No. 4,732,831, discloses an improved xeroprinting process that employs a master having a photohardenable coating on a conductive substrate. The coating contains an organic polymeric binder, an ethylenically unsaturated monomer, and a photoinitiator system. When the master is imagewise exposed to actinic radiation, the exposed regions polymerize. These exposed regions exhibit significantly higher resistance than the unexposed regions. When the exposed master is used in a xeroprinting process, the polymerized regions hold electrical charge and are developed by toner. The unpolymerized regions discharge to ground through the conductive backing and, therefore, do not attract toner.
Liquid electrostatic toners or developers, which employ high-purity isoparaffinic hydrocarbons as the liquid, can be used in xeroprinting processes. If the unpolymerized monomer present in the master is soluble in the liquid, it will be leached from the unexposed regions as multiple copies are produced. These regions will become insulating due to removal of the monomer. Consequently, copy quality will deteriorate with each succeeding copy.
Research Disclosure 294, 29464 (October, 1988) discloses that triethanolamine triacrylate and diethanolaniline diacrylate are not readily leached by isoparaffinic hydrocarbon solvents. However, this approach requires that specialized monomers be prepared. Detig et al., U.S. Pat. No. 4,859,557, discloses a photopolymer master in which the photopolymer material is protected by a transparent overcoat material whose surface is hard and has low friction. The transparent overcoat is not conducting so only low resolution images (4 to 5.6 line pairs per mm) were obtained.
A need exists for a high resolution, photohardenable electrostatic master from which multiple copies can be produced without deterioration of copy quality and which can be prepared from readily available monomers.